The present invention relates to a method and apparatus for determining symbol alignment in Discrete Multi-Tone (DMT) or similar communications systems and a system incorporating the same.
Telecommunication systems that interconnect wireline subscriber terminals are being developed to support broadband data communication. More particularly, recent developments in broadband communications protocols allow broadband data to be overlaid on narrowband voice or integrated service digital network (ISDN) traffic. Specifically, the interconnection of broadband modems located at the subscriber terminal and at an exchange allow current broadband access systems to communicate on spare spectrum (i.e. spare frequency channels) of a twisted pair communication resource; the spare frequency channels being isolated from conventionally encoded voice signals by a suitable filter. In this respect, and depending upon the complexity of the xDSL coding scheme, overlaid broadband systems can support data rates in excess of two Megabits per second (Mbps), although this rate is dependent upon the physical parameters of the connection, e.g. the overall length of the twisted pair and its composition and configuration.
Asymmetric Digital Subscriber Line (ADSL) and High-Speed digital Subscriber Line (HDSL) protocols, for example, can support data rates of 2 Mbps over distances of approximately three kilometres, while more complex schemes (such as VDSL) can support data rates of 8 Mbps and above over distances of, typically, less than two kilometres. Protocols such as Very high-speed Digital Subscriber Line (VDSL) utilise multiple sub-channel carriers, e.g. in a discrete multi-tone (DMT) environment, to provide a system that mitigates the effects of cross-talk by selectively ignoring noise-affected sub-channel carriers or reducing the number of bits supported on each sub-channel. DMT provides a comb of frequency carriers that are each separately modulated and then combined to generate a composite signal envelope. As such, information (both control information and traffic) is distributed across a number of different frequency carriers.
DMT schemes for supporting VDSL are often realised in a time division duplex (TDD) transmission environment in which a single communication resource (i.e. a frequency band) supports both up-link and down-link transmissions using the same frequencies. In other words, there is a sharing in time of the bandwidth provided by the extended spectrum. The use of guard periods between adjacent groups of time-slots within a TDD frame ensures that rogue overlapping transmissions within the up-link and down-link do not occur, and hence eliminates the likelihood of near-end cross talk (NEXT).
In order to establish effective end-to-end communication in a communication system, it is necessary for synchronisation between a transmitting unit and an interconnected receiving unit to occur. This is true for both a radio frequency environment and a wireline environment, such as a VDSL system employed over a twisted pair. More specifically, synchronisation is required to demodulate encoded signals that are addressed to the receiver. In this respect and in relation to a DMT system (or the like, such as an orthogonal frequency division multiplexed OFDM scheme), a pilot carrier or tone is used in a training sequence on a dedicated (pre-allocated) sub-channel. Initially, upon receipt of the pilot tone, the receiver acquires frequency lock and then establishes phase lock.
The invention seeks to provide an improved method and apparatus for determining symbol alignment in discrete multi-tone communications systems and a system incorporating the same.
According to a first aspect of the present invention there is provided a method of determining symbol alignment in a discrete multi-tone communications system comprising the steps of: receiving a signal on each of at least one selected carrier frequency; for each of a plurality of receiver window alignments with respect to said signals, deriving a measure of received signal power level on each said selected carrier frequency; for each said receiver window alignment and selected carrier frequency deriving a measure of spectral leakage on at least one adjacent carrier frequency; selecting a receiver window alignment responsive to said derived measures.
Advantageously this enables faster and more accurate selection of a receiver window alignment for symbol extraction than making the selection based on the power levels of the selected carrier frequency alone.
Advantageously the use of a power measurement on said selected carriers and said adjacent carriers provides a more selective metric for the symbol alignment process than is obtained from a power measurement on said selected carrier frequency alone.
Preferably, said at least one adjacent carrier frequency is a silent carrier frequency.
Preferably, silent carrier power levels are at least in the order of 30 dB lower than active carrier power levels.
Preferably, said selection of receiver window alignment is responsive to the difference between the received signal power level on said selected carrier frequency and a multiple of said received signal power level on said adjacent carrier frequencies
Preferably, said multiple is in the order of 50.
Preferably, each said measure of received signal power levels is a time average of a plurality of measures of received signal power levels.
Advantageously, this provides a more representative carrier power measurement and hence improves the reliability of the received symbol alignment.
In a first preferred embodiment, said selected receiver window alignment is one of said set of receiver window alignments.
In a second preferred embodiment, said selected receiver alignment window is interpolated between alignments in said set of receiver window alignments.
Advantageously this permits use of a smaller number of receiver window alignments during sampling whilst still obtaining a well-positioned receiver window alignment.
Preferably, said carrier frequencies are in a frequency band 0-10 MHz.
Advantageously this permits the method to be used over existing twisted copper pair to the home.
Preferably, successive selected carrier frequencies are separated by a plurality of silent carrier frequencies.
This reduces the likelihood of a given silent carrier frequency being affected by more than one active carrier.
Preferably, selected carrier frequencies are distributed across a frequency band utilised by said discrete multi-tone communications system.
Advantageously, this assists both in selecting a compromise symbol alignment suitable for all discrete frequencies in the frequency band, and in reducing the susceptibility to locally poor signal to noise ratio in any individual frequency.
Preferably, the step of selecting a receiver window alignment responsive to said derived measures comprises the steps of: identifying which receiver window alignment gives rise to the largest difference; searching backwards and forwards to identify receiver window alignments at which said differences drop below a given fraction of said largest difference; selecting an optimal receiver window alignment between said receiver window alignments at which said differences drop below a given fraction of said largest difference.
Advantageously this compensates for cases where the largest difference may be unduly influenced by received noise which would give rise to a suboptimal choice of alignment.
Preferably, said receiver window alignment between said receiver window alignments is chosen to be approximately mid-way between said receiver window alignments at which said differences drop below a given fraction of said largest difference.
Preferably, said fraction is approximately nine tenths.
Preferably, said signal forms part of a TDD communications channel comprising a succession of frames, each frame comprising a receive region and each comprising a succession of DMT symbols.
Preferably, said step of deriving measures of received signal power level on each of said selected carrier frequencies is performed on a single symbol in each said receive region.
In one preferred embodiment, the step of selecting a receiver window alignment responsive to said derived measures comprises the steps of: selecting a narrower range of receiver window alignments responsive to said derived measures; selecting, within said narrower range, a second range of receiver window alignments with respect to said signals; for each said receiver window alignment in said second range deriving a further measure of received signal power level on each said selected carrier frequency; for each said receiver window alignment in said second range and selected carrier frequency deriving a further measure of spectral leakage on at least one adjacent carrier frequency; selecting a receiver window alignment responsive to said further derived measures.
Advantageously the approach of performing a coarse grained alignment followed by a finer grained alignment may be used to speed up the alignment process.
According to a second aspect of the present invention there is provided a signal receiver arranged to receive discrete multi-tone signals and comprising: a signal input arranged to receive a signal on each of at least one selected carrier frequency; a processor arranged to select a plurality of receiver window alignments with respect to said signals, for each said receiver window alignment to derive a measure of received signal power level on each said selected carrier frequency, for each said receiver window alignment and selected carrier frequency to derive a measure of spectral leakage on at least one adjacent carrier frequency and to select a receiver window alignment responsive to said derived measures.
The invention also relates to a discrete multi-tone system comprising: a receiver according to the second aspect of the present invention; a transmitter having a signal output and arranged to provide said signals and to remain silent on said adjacent carrier frequencies; and a transmission medium arranged to convey discrete multi-tone signals from said output port to said input port.
The invention also relates to a TDD communications system comprising a signal receiver according to the second aspect of the present invention.
The invention also relates to a VDSL discrete multi-tone modem comprising a signal receiver according to the second aspect of the present invention.
The invention also relates to a telecommunications network comprising a signal receiver according to the second aspect of the present invention.
According to a further aspect of the present invention there is provided a program for a computer on a machine readable medium arranged to perform the steps of: receiving a signal on each of at least one carrier frequency; applying each of a plurality of receiver window alignments to said signals; deriving a measure of each said receiver window alignment as applied to each said signal; for each said receiver window alignment deriving a measure of spectral leakage on a carrier frequency adjacent at least one of said at least one carrier frequencies; selecting a receiver window alignment responsive to said derived measures.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.